GRE TUNNEL Question

[NC16390371]

We have a point to point GRE tunnel which has some issues that I would like to see if I can get some guidance.  When clients connect to the WLAN and are routed thru this tunnel they experience latency on some https applications.  However if you change the MTU on the  laptops 802.11 radio from 1500 over to 1476 the problem disappears.  What is the best approach to change this without changing it on every machine.

Traffic Flow is as follows:

Client --> LWAPP --> (TUNNELED) --> WLAN Controller --> VLAN INTERFACE --> TUNNEL <<WAN>> TUNNEL --> <<INTERNET>>                        

[Joseph W. Doherty]

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Posting

If your WLCs support it, enable mss adjust, globally.

If your WAN GRE tunnels support it, set IP MTU to 1476, mss adjust to 1436 and enable tunnel PMTUD.

[Richard Burts]

The bytes added by GRE result in a frame that sometimes is larger than the permitted MTU. I have found that frequently using the command ip tcp adjust-mss is an effective way to avoid the problem of fragmenting traffic that is going through GRE. I generally apply it on the LAN interface but I have seen some people apply it on the tunnel interface.

HTH

Rick

[paul driver]

Hello

In regards to Joesph and Richard comments

int tun xx

ip mtu 1476

ip tcp adjust-mss 1436

res

Paul

[Joseph W. Doherty]

Disclaimer

The  Author of this posting offers the information contained within this  posting without consideration and with the reader's understanding that  there's no implied or expressed suitability or fitness for any purpose.  Information provided is for informational purposes only and should not  be construed as rendering professional advice of any kind. Usage of this  posting's information is solely at reader's own risk.

Liability Disclaimer

In  no event shall Author be liable for any damages whatsoever (including,  without limitation, damages for loss of use, data or profit) arising out  of the use or inability to use the posting's information even if Author  has been advised of the possibility of such damage.

Posting

And to add to Paul's actual tunnel interface configuration statements . . .

tunnel path-mtu-discovery

[NC16390371]

One last question.... can I do the following :

interface vlan xxx

ip mtu 1476

ip tcp adjust-mss 1436

interface tunnel xx

tunnel path-mtu-discovery

I would rather change it on the VLAN interface before it hits the tunnel.

[Joseph W. Doherty]

Disclaimer

The  Author of this posting offers the information contained within this  posting without consideration and with the reader's understanding that  there's no implied or expressed suitability or fitness for any purpose.  Information provided is for informational purposes only and should not  be construed as rendering professional advice of any kind. Usage of this  posting's information is solely at reader's own risk.

Liability Disclaimer

In  no event shall Author be liable for any damages whatsoever (including,  without limitation, damages for loss of use, data or profit) arising out  of the use or inability to use the posting's information even if Author  has been advised of the possibility of such damage.

Posting

You might be able to do that, but for different reasons, you want to apply to the tunnel interfaces.

Regarding your question about whether needed on both ends, MTU and PMUD yes, adjust-mss actually no, but normally you place it on both ends too.  (Adjust-mss applies to traffic in both directions.)

[paul driver]

hello

i would apply it on the tunnel if thats where your traffic is traversing

res
paul

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[NC16390371]

Can I assume we need these on both ends of the tunnel ?

[paul driver]

hello

yes apply both ends

res
paul

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[paul driver]

@ Joseph

wasnt aware of that -
cheers
paul


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